This invention relates to a high-voltage terminal bushing for electrical apparatus and, more particularly, relates to means for improving the ability of such a bushing to withstand impulse voltages without external flashover.
A typical high-voltage terminal bushing comprises a tubular shell of electrical insulating material containing a central passageway extending between opposite ends of the bushing and an electrical conductor extending between said ends via the passageway. In the type of bushing that I am concerned with, there is a cylindrical gap containing gaseous dielectric, such as air, between the conductor and the internal wall of the central passageway. Such bushings must be able to withstand relatively high values of impulse voltage. For example, according to American National Standard C37.06-1971, a bushing for an outdoor circuit breaker having a rated maximum voltage of 15.5 kV rms must be able to withstand a full wave impulse voltage of 110 kV and a 2 microsecond chopped wave impulse voltage of 142 kV, the impulse tests being made with a 1.2.times.50 microsecond wave. These values of impulse voltage, in part, constitute what is commonly referred to as the rated impulse insulation level of the bushing. In working with certain bushings of the above type, I have encountered flashovers along the external surface of the bushing when impulse voltages approaching the rated impulse insulation level were applied between the conductor and ground. Conventional approaches for inhibiting such external flashovers are to lengthen the external surfaces of the insulating shell, or to change their configuration, or to change the shape of the electrodes at the boundaries of these surfaces. Such approaches involve extensive modification and redesign of the bushing and tend to be rather expensive.